Most window sash balances for counteracting the weight of window sashes are now made with springs. For example, springs exhibiting resilience in the form of either tension or torsion are commonplace in window balances.
Tension springs are generally more effective for exerting lifting forces needed to counteract heavy window sash weight, but the lifting force exerted by such tension springs tends to vary significantly with sash travel, i.e., with the amount the spring is extended. Friction opposing sash movement is controlled to minimize so-called sash "hop" and "drop" at the opposite ends of sash travel.
Torsion springs are often preferred for exerting lesser lifting forces because the torsion springs can be arranged to exert a more constant lifting force over the course of sash travel. Torsional spring force is converted into a lifting force by an elongated spiral member that engages a follower attached to one end of the spring. Although the amount of torque exerted on the spiral member increases with an increasing amount of relative travel between the spiral member and spring, the pitch of the spiral member is varied so that the amount of lifting force exerted by the spiral member remains relatively constant throughout its length of travel.
Tension and torsion springs have also been used together to provide substantially more lifting force than torsion springs while maintaining a more constant lifting force than tension springs. Generally, the torsion spring is mounted within the tension spring, and the variation in pitch along the length of the spiral member is increased to help compensate for the variation in force exerted by the tension spring at different lengths of extension.
For example, there are commercially available balances that include a torsion spring mounted within a tension spring. However, the two springs tend to interfere with each other so that neither spring works exactly as intended, and the available balances tend to perform inconsistently. A much older design found in U.S. Pat. No. 2,041,646 to A. Larson uses a rigid tube to separate the torsion and tension springs so that neither spring interferes with the other. However, similar to the commercially available designs, the balance of Larson is difficult to set in pretension and does not readily accommodate shoe attachments used in tilt and take-out windows.